1. Field of the Invention
The present invention relates to a method for forming a functional film, in particular a functional deposited film applied to electronic devices such as semiconductor devices, photosensitive devices for electrophotography, and optical input sensor devices for an apparatus to which optical images are inputted.
2. Related Background Art
Up to now, a functional film composed of amorphous or polycrystalline structure, such as semiconductor films, insulating films, photoconductive films, magnetic films or metal films has conventionally been formed according to an individual, suitable film-forming method in the light of desired physical properties or applications thereof.
As methods for forming deposited films, there have conventionally been attempted vacuum deposition method, plasma CVD method, thermal CVD method, photo-CVD method, reactive sputtering technique, ion-plating method, etc. and the Plasma-CVD technique is, in general, widely utilized and industrialized.
However, there still remains room for synthetically improving these deposited films, formed according to the methods as listed above, in particular in their properties such as electrical, optical properties, fatigue property in the case where it is used repeatedly and resistance to environment as well as productivity and mass productivity inclusive of uniformity and reproducibility, since it has recently been intended that these deposited films are applied to electronic devices and photoelectronic devices which are expected to have more improved higher functions.
The reaction processes for forming a deposited film according to a conventional Plasma CVD method widely utilized are quite complicated compared with the conventional so-called Thermal CVD method and further there are not a few vague points in its reaction mechanisms.
Moreover, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of introduced gases, pressure during the formation of the film, high-frequency power, electrodes structure, reaction vessel structure, evacuation rate, and plasma generating system, etc.) Because of the combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences were exerted frequently on the deposited film formed. In addition to those mentioned above, the parameters specific to such apparatuses for depositing functional films should be selected depending on each of them and therefore, it has practically been quite difficult to generalize the conditions for film-forming processes or apparatuses.
In particular, it has practically been believed to be optimum that a film of III-V compound semiconductor should be deposited according to Plasma-CVD method. This is because this method makes it possible to impart sufficient electric and optical properties suitable for each applications to the resulting deposited films.
However, a vast facility investment is required to establish an apparatus which permits the mass production of deposited films when the deposited film is formed according to Plasma-CVD method, since it is sometimes required to achieve mass productivity and a good reproducibility while sufficiently satisfying the requirements with respect to the enlargement of deposited area, uniformity in the film thickness, and uniformity in the quality of the deposited film. Furthermore, items of control required to attain mass production become complicated, tolerance of control becomes narrower and a delicate adjustment of the apparatus for depositing functional films is required. Under these circumstances, it has been pointed out that these problems should effectively be solved in the future.
On the other hand, a conventional technique principally relying on the well known CVD technique requires a relatively high temperature to form a deposited film and seldom provides a deposited film satisfying the requirements of properties which are commercially acceptable.
These problems accompanied by the conventional film-forming techniques still remain unsolved and these problems have been expected to be eliminated. The elimination of them is quite important, in particular, in producing an electrically conductive thin film composed of aluminum (Al)-, molybdenum (Mo)-, tungsten (W)-, titanium (Ti)-, or tantalum (Ta)-containing compounds.
As already explained above, it has eagerly been expected, in formation of functional films, to develop a method for forming a deposited film, which is capable of mass production by means of an apparatus of low cost, while maintaining the practically acceptable properties of the deposited film and the uniformity thereof.